Adaptive immune responses in the gut are initiated in discretely organized collections of lymphocytes and antigen-presenting cells closely associated with the mucosal epithelial lining of the bowel and in mesenteric lymph nodes (see Fig. 1). Naive lymphocytes are exposed to antigens in these sites and differentiate into effector cells. These gut-associated lymphoid tissues adjacent to the mucosal epithelium are sometimes referred to as GALT, which is the gastrointestinal version of MALT, although the terms are often used interchangeably. The most prominent GALT structures are Peyer’s patches, found mainly under the epithelium in the distal ileum, but there are many lymphoid follicles, either isolated or in small aggregates, in the lamina propria of the appendix and colon. Peyer’s patches have the structure of lymphoid follicles, with germinal centers containing B lymphocytes, T follicular helper cells, follicular dendritic cells, and macrophages. The germinal centers in the follicles are surrounded by naive follicular B cells expressing IgM and IgD. Between the follicles are T cell–rich parafollicular areas, similar to lymph nodes; overall, the ratio of B cells to T cells in GALT is about five times higher than in lymph nodes. The subepithelial dome region, located between the follicles and the overlying epithelium, contains B and T lymphocytes, DCs, and macrophages, which are exposed to antigens transported across the epithelium by M cells, described below. Distinct from lymph nodes, GALT structures are not encapsulated, and antigen is delivered directly to these structures, independent of lymphatics. Development of the specialized lymphoid structures, such as Peyer’s patches and isolated follicles in the gut lamina propria, requires lymphoid tissue inducer cells, which are a subset of ILC3s that produce the cytokine lymphotoxin-β (LTβ).
Fig1. The gastrointestinal immune system. (A) Schematic diagram of the cellular components of the mucosal immune system in the intestine. The main features include an epithelial barrier covered by secreted mucus, dendritic cells (DCs) and microfold (M) cells that sample antigens, Tuft cells that respond to helminths by secreting cytokines, various innate sentinel cells, and lymphocytes in the lamina propria beneath the epithelial layer, organized mucosal-associated lymphoid tissues beneath the epithelial barrier, such as Peyer’s patches, draining mesenteric lymph nodes, and plasma cells beneath the epithelium that secrete immunoglobulin A (IgA), which is transported into the lumen. (B) Photomicrograph of mucosal lymphoid tissue in the human intestine. Similar aggregates of lymphoid tissue are found throughout the gastrointestinal tract. ILC, Innate lymphoid cell.
Antigens are delivered from the lumen to the GALT through specialized cells within the gut epithelium called M cells (Fig. 2). M cells are located in regions of the gut epithelium called follicle-associated (or dome) epithelium that overlie the domes of Peyer’s patches and other GALT structures. Although M cells and the more numerous absorptive epithelial cells likely arise from a common epithelial precursor, the M cells are distinguishable by a thin glycocalyx, relatively short, irregular microvilli (referred to as microfolds), and large fenestrations in their membranes, all features that enhance the uptake of antigens from the gut lumen. Unlike the absorptive epithelium, the follicle-associated epithelium where M cells are located has a paucity of both mucus-secreting goblet cells and defensin-secreting Paneth cells and reduced ability to trans port IgA into the lumen. These features of M cells facilitate their close association with luminal microbial antigens. The main function of M cells is transcellular transport of various substances from the lumen of the intestine across the epithelial barrier to underlying antigen-presenting cells. M cells take up luminal contents efficiently and in various ways, including phagocytosis in a manner similar to macrophages, and vesicular endocytosis or fluid-phase pinocytosis. M cells express surface molecules that bind microbial structures and pro mote their uptake; one example is glycoprotein 2, which binds type I pili on gram-negative bacteria in the gut and mediates uptake and delivery of these bacteria to Peyer’s patches. These pathways enable uptake of whole bacteria, viruses, and soluble microbial products. Unlike macrophages or DCs, M cells do not engage in extensive processing of the substances they take up, but rather move the particles and molecules through endocytic vesicles across the cytosol and deliver them by exocytosis at the basolateral membrane to DCs or B cells in the dome regions of underlying Peyer’s patches and lamina propria lymphoid follicles. Although M cells play an important role in protective immunity to luminal microbes, some microbes have evolved to take advantage of M cells as a route of invasion through the mucosal barrier. The best described example of this is Salmonella typhimurium, which is similar to Salmonella typhi, the cause of typhoid fever. These bacteria bind specifically to lectins expressed on M cells and are internalized. The bacteria are toxic to the M cells, producing gaps in the epithelium that promote invasion of more organisms. M-cell lectins also may be used by certain enteric viruses to breach the epithelial barrier.
Fig2. Microfold (M) cells in the small intestine. M cells are specialized intestinal epithelial cells found in the small bowel epithelium overlying Peyer’s patches and lamina propria lymphoid follicles (A). Unlike neighboring absorptive epithelial cells with tall microvilli, M cells have short villi and appear sunken in the scanning electron microscopic image shown in (B). M cells transport intact microbes or molecules across the mucosal barrier into gut-associated lymphoid tissues, where they are handed off to dendritic cells (C). B, From Dillon A, Lo DD. M cells: intelligent engineering of mucosal immune surveillance. Front Immunol. 2019;10:1499.
Mesenteric lymph nodes collect lymph-borne antigens from the small and large intestines and are sites of differentiation of effector and regulatory lymphocytes that home back to the lamina propria. There are 100 to 150 of these lymph nodes in the mesentery. Mesenteric lymph nodes serve some of the same functions as GALT, including differentiation of B cells into IgA secreting plasma cells and the development of effector T cells as well as regulatory T cells. The cells that differentiate in the mesenteric lymph nodes in response to bowel wall invasion by pathogens or commensals often home to the lamina propria.
Lingual and palatine tonsils are unencapsulated lymphoid structures located beneath stratified squamous epithelial mucosa in the base of the tongue and oropharynx, respectively, and are sites of immune responses to microbes in the oral cavity. These tonsils, together with nasopharyngeal tonsils (also called adenoids), form a ring of lymphoid tissue called Waldeyer’s ring. The bulk of the tonsillar tissue is composed of lymphoid follicles, usually with prominent germinal centers. The lingual and palatine tonsils are separated from the microbe-rich oral cavity by multiple layers of squamous epithelial cells, rather than the single columnar epithelial cell layer that separates the intestinal lumen from other GALT. There are numerous narrow and deep invaginations of the surface squamous epithelium, called crypts, which grow into the tonsillar follicular tissue. The lingual and palatine tonsils respond to infections of the epithelial mucosa by significant enlargement and vigorous, mainly IgA, antibody responses. Typically, tonsillar enlargement is caused by infection with streptococci and the Epstein-Barr virus, often in children.
Effector lymphocytes that are generated in the GALT and mesenteric lymph nodes are imprinted with selective integrin- and chemokine receptor–dependent gut-homing properties, and they circulate from the blood back into the lamina propria of the gut (Fig. 3). The functions of the gastrointestinal immune system depend on a large number of T cells and antibody-secreting cells that are able to recirculate back into the lamina propria and respond rapidly to pathogens. Both effector T cells and IgA-secreting B cells acquire this gut-homing phenotype because of changes in adhesion molecules and chemokine receptors that are acquired during lymphocyte activation in the GALT or gut-draining lymph nodes. The major integrin on gut-homing B and T lymphocytes is α4 β7, which binds to the MAdCAM-1 protein expressed on postcapillary venular endothelial cells in the gut lamina propria. Gut homing requires the chemokine receptor CCR9 on the B and T lymphocytes and its chemokine ligand CCL25, which is produced by intestinal epithelial cells. The combined expression of MAdCAM-1 on endothelium and CCL25 in tissues is restricted to the gut. Homing of IgA-producing cells to the colon also requires CCR10 expression and the chemokine CCL28, but this is not a gut-specific pathway because CCL28 is expressed by epithelial cells in other mucosal tissues, such as the lung and genitourinary tract. Blocking monoclonal antibodies that are specific for the α4 chain of α4 β7 have been used to treat patients with inflammatory bowel disease (IBD) based on the knowledge that effector T cells use this integrin to enter gut tissues.
Fig3. Homing properties of intestinal lymphocytes. The gut-homing properties of effector lymphocytes are imprinted in the lymphoid tissues, where they have undergone differentiation from naive precursors. Dendritic cells in gut-associated lymphoid tissues (GALT), including Peyer’s patches and mesenteric lymph nodes, are induced by cytokines such as thymic stromal lymphopoietin (TSLP) and other factors to express retinaldehyde dehydrogenase (RALDH), which converts dietary vitamin A into retinoic acid. When naive B or T cells are activated by antigen in GALT, they are exposed to retinoic acid produced by the dendritic cells, and this induces the expression of the chemokine receptor CCR9 and the integrin α4 β7 on the differentiated plasmablasts and effector T cells. The effector lymphocytes enter the circulation and home back into the gut lamina propria because the chemokine CCL25 (the ligand for CCR9) and the mucosal addressin cell adhesion molecule 1 (MAdCAM -1) (the ligand for α4 β7 ) are displayed on lamina propria venular endothelial cells.
The gut-homing phenotype of IgA-producing B cells and effector T cells is imprinted by DCs through the action of retinoic acid during the process of T-cell activation (see Fig. 3). In addition to promoting naive T-cell differentiation into effector T cells and naive B-cell differentiation into IgA antibody secreting cells (discussed later), DCs in GALT and mesenteric lymph nodes also provide signals that lead to the expression of the α4 β7 integrin and CCR9 on these effector cells. The induction of these homing molecules depends on secretion of retinoic acid by the DCs. Gut lymphoid tissues are exposed to dietary vitamin A, and DCs in GALT and mesenteric lymph nodes express retinaldehyde dehydrogenase (RALDH), the enzyme needed for retinoic acid synthesis from vitamin A, whereas DCs in other tissues do not. In addition, intestinal epithelial cells express RALDH and can synthesize retinoic acid. How retinoic acid induces expression of gut-homing molecules is not known. Consistent with these properties of the intestinal immune system, it is known that oral vaccination favors the expansion of gut-homing IgA-producing B cells as compared with intradermal immunization.
The lamina propria contains diffusely distributed effector lymphocytes, DCs, and macrophages and is the site of the effector phase of gastrointestinal adaptive immune responses. As discussed previously, effector lymphocytes generated in Peyer’s patches, other GALT structures, and mesenteric lymph nodes home back into the lamina propria. In this location, T cells can respond to invading pathogens, and B cells can secrete antibodies that are transported into the lumen and neutralize pathogens before they invade.
